1. Field of the Invention
The present invention relates to a sheet processing apparatus, an image forming system, and a method of additionally folding a sheet bundle, and particularly relates to a sheet processing apparatus having a function of performing folding processing on a sheet-shaped recording medium such as paper, recording paper, and transfer paper (hereinafter simply referred to as a “sheet” in this specification), an image forming system including a sheet processing apparatus, and a method of additionally folding a sheet bundle executed by the sheet processing apparatus.
2. Description of the Related Art
A conventional post-processing apparatus used in combination with an image forming apparatus such as a copying machine binds a saddle stitched booklet by stitching a sheet or a plurality of sheets at its center part and folding the same at the center part with a pair of folding rollers arranged parallel to the direction of sheet folding. In addition, a technique is already known that carries out additional folding with a roller moving along the back of a saddle stitched booklet so as to strengthen a fold of the booklet.
Such an additionally folding technique performs additional folding on the back (a fold part) of a booklet (a sheet bundle) by putting a roller standing by on the outside of the booklet on the back of the booklet and moving the roller.
Known examples of this kind of additionally folding technique are disclosed in Japanese Patent Application Laid-open No. 2008-207964 and Japanese Patent Application Laid-open No. 2009-126685.
Japanese Patent Application Laid-open No. 2008-207964 describes a folding means for performing folding processing on a sheet or a sheet bundle with a plurality of bound sheets and a fold strengthening mechanism for strengthening a fold of the sheet or the sheet bundle folded by the folding means by pressing the fold. The fold strengthening mechanism and the folded sheet(s) move relatively to each other along the fold and intermittently stop during the move.
Japanese Patent Application Laid-open No. 2009-126685 describes a sheet folding device that includes folding means for performing folding processing on a sheet-shaped recording medium that is carried in and additional folding means for performing additional folding after the folding processing by the folding means by moving on a fold of the sheet-shaped recording medium in a direction perpendicular to a sheet-ejecting direction and pressurizing the fold. The additional folding means is inclined from the normal of a medium surface of the sheet-shaped recording medium in a direction of force generated in a moving direction when the fold is pressurized.
According to Japanese Patent Application Laid-open No. 2008-207964, the fold strengthening mechanism and the folded sheet(s) relatively move along a fold and intermittently stop during the move. Because of this, strength is increased with regard to additional folding; however, productivity is conversely decreased.
According to Japanese Patent Application Laid-open No. 2009-126685, a direction for applying an energizing force is inclined from the moving direction of an additional folding mechanism for the purpose of load reduction. In this case, because the additional folding mechanism consists of a fixed receiver opposed to a pressurizing roller, a pressuring force against a sheet bundle is generated in the thickness direction of the sheet bundle. This requires a stiffer device, which results in increased size and cost of the apparatus.
Therefore, there is a need for a sheet processing apparatus capable of performing additional folding on a fold part of a folded sheet bundle without reducing productivity or increasing the size or cost of the apparatus.